Conventional implantable leads for use with implantable electrical devices such as cardiac pacemakers and defibrillators are typically constructed of a helically wound conductor having an outer insulation layer of tubular form surrounding the wire helix. The tubular insulation is most commonly of an elastomeric material such as silicone or polyurethane. The combination of a helically wound conductor with elastomeric outer insulation provides these conventional constructions with a substantial amount of potential elastic deformation in the direction of the length of the lead.
The fundamental requirements of implantable leads are that they must have excellent mechanical integrity, electrical insulating properties and biocompatibility, and must be flexible with a long flex life to accommodate attachment to a beating heart.
Conventional implantable leads have several disadvantages. The silicone or polyurethane outer coverings are not ideally biocompatible and are frequently known to provoke adverse tissue reactions over time. Polyurethane outer coverings are known to crack under stress. Silicone outer coverings are vulnerable to abrasion over time. Additionally, these conventional leads are known to break during attempts to remove them from implanted patients by the application of a tensile force. In these cases the remaining portion must be abandoned within the patient's body or must be surgically removed.
Implantable lead wires using insulation materials other than the conventional silicones or polyurethanes have been described previously. U.S. Pat. No. 4,573,480 describes an implantable electrode lead in the form of a helically wound conductor having a tubular insulating layer surrounding the helically wound wire wherein the tubular insulating layer is porous polytetrafluoroethylene (hereinafter PTFE) having a pore size limited to a maximum size described as "being essentially impervious to body fluids to prevent tissue growth thereon." This pore size is described as being not larger than 4 microns. While pore sizes of this range and smaller are known to preclude cellular ingrowth, the material remains pervious to body fluids which will wet out such an insulating layer shortly after implantation. The result is that the effectiveness of the electrical insulation is destroyed. Alternatively, this patent teaches that the tubular porous PTFE insulating layer may be provided with an outer covering of smooth and impervious material. While this alternative construction prevents the wetting out of the porous PTFE layer by body fluids, it loses the biocompatible advantage provided by the tissue contacting outer surface of porous expanded PTFE.